The Navstar Global Positioning System (GPS) is used to determine exact geographic position (i.e., latitude, longitude and height above the earth) absolute velocity, as well as the exact time. The navigation device, receiver, must calculate the position velocity and the time by determining distance and relative velocity to a series of satellites. The velocity of the receiver is calculated from the doppler frequency shift of signals transmitted from space while the exact position of the receiver is calculated from the time shift of data due to the distance the signals must travel from the satellite. This distance is called range and the doppler shift yields range rate.
A GPS receiver must receive signals generated from a satellite about 11,000 miles away. A GPS satellite transmits about 6 watts spread spectrum signal. The satellite and receiver, employ spread spectrum techniques to differentiate the signal from the noise. "Spread spectrum" means that as time passes, the frequency at which a signal is being transmitted will change or the instantaneous phase at which the signal is being transmitted will change. Using correlation techniques, the satellite receiver can match the spread spectrum signal coming from the satellite with an image of the signal that the receiver attempts to estimate. A precise match of the satellite's spread spectrum signal produces a potential signal processing gain of up to 53 decibels. The use of spread spectrum techniques is essential to receive 6 watts of energy transmitted 11,000 miles away. At the antenna, the GPS signal is about 20 dB below ambient cosmic noise.
Using the GPS system, four transmitting satellites are required, to solve for the three spatial dimensions and time. The basic method of determining position is knowing the time difference from each of the satellites. The time difference for each satellite is the time required for a signal initiated at the satellite to be received by the user. Therefore, at least four satellites must be tracked. This is accomplished ideally by the use of at least four hardware receiver channels.
Classically and historically, receiver channels have been large, power hungry and expensive. While this gives the best performance, it costs a lot of money. The lowest cost approach uses sequential tracking. Under sequential tracking, there is one receiver channel that sequences across multiple satellites--tracking each satellite for a predetermined period of time, and then tracking another satellite, etc. Sequential tracking requires the minimum hardware, but also has the lowest performance characteristics.
There has been developed a multiplex technique which was essentially a compromise between using four receiver channels and using purely sequential tracking. This technique sequences very quickly across four satellites. This provides the dynamic capability of a continuous track receiver, but has a serious signal strength penalty under jamming conditions. The present state of the art requires a choice between either large and expensive receivers for high performance, or small and more affordable sequential tracking, with a severe compromise in performance.
Therefore, a need exists for a receiver that is small and affordable, yet with no compromise in performance.
A further need exists in the art for a GPS receiver that achieves enhanced performance under diabolical conditions, such as jamming and fast acquisition.
A still further need exists in the art for a receiver having simultaneous C/A and P or Y-code (P(Y)-code) capability while reducing the parts count and enhancing signal acquisition time.